1. Field of the Invention
The present invention relates to an on-site system and a method for providing ultra-high-purity nitric acid to a point of use. The present invention also relates to an on-site subsystem and a method, in a semiconductor device fabrication facility, for providing ultra-high-purity nitric acid to a semiconductor manufacturing operation.
2. Description of the Related Art
a. Contamination Control
Contamination is generally an overwhelmingly important concern in integrated circuit (IC) manufacturing. A large fraction of the steps used in modern integrated circuit manufacturing are cleanup steps of one kind or another. Such cleanup steps are used, for example, to remove organic contaminants, metallic contaminants, photoresist (or inorganic residues thereof), byproducts of etching, native oxides, etc.
As of 1995, the cost of a new front end (IC wafer fabrication facility) was typically more than one billion dollars ($1,000,000,000). A large fraction of the cost for such facilities is directed to measures for particulate control, cleanup, and contamination control.
One important and basic source of contamination in semiconductor fabrication is impurities in the process chemicals. Since the cleanup steps are performed so frequently in and are so critical to IC fabrication, contamination due to cleanup chemistry is very undesirable.
b. Wet Versus Dry Processing
One of the long-running technological shifts in semiconductor processing has been the changes (and attempted changes) between dry and wet processing. In dry processing, only gaseous or plasma-phase reactants come in contact with the wafer or wafers being treated. In wet processing, a variety of liquid reagents are used for a multitude of purposes, such as the etching of silicon dioxide, silicon nitride and silicon, and the removal of native oxide layers, organic materials, trace organic or inorganic contaminants and metals.
While plasma etching has many attractive capabilities, it is not adequate for use in cleanup processes. There is simply no available chemistry with plasma etching to remove some of the most undesirable impurities, such as gold. Thus, wet cleanup processes are essential to modern semiconductor processing, and are likely to remain so for the foreseeable future.
Plasma etching is performed using a photoresist mask in place, and is not immediately followed by high-temperature processes. After plasma etching, the resist is stripped from the wafer surface using, for example, an O.sub.2 plasma treatment. Cleanup of the resist stripped wafer(s) is then necessary.
The materials which the cleanup process should remove include, for example, photoresist residues (organic polymers), sodium, alkaline earth metals (e.g., calcium, magnesium) and heavy metals (e.g., gold). Many of these contaminants do not form volatile halides. As a result, plasma etching will not remove such contaminants from the wafer surface. Hence, cleanup processes using wet chemistries are required.
Because any dangerous contaminants stemming from the plasma etching process are removed prior to high-temperature processing steps by wet chemical treatment, the purities of plasma etching process chemicals (i.e., liquified or compressed gases) are not as critical as those of the liquid chemicals used in cleanup processes. This difference is due to the impingement rate of the liquid chemical at the semiconductor surface typically being one million times greater than that of the plasma species in plasma etching. Moreover, since the liquid cleanup steps are directly followed by high-temperature processes, contaminants on the wafer surface tend to be driven (i.e., diffused) into the wafer.
Wet processing has a major drawback insofar as ionic contamination is concerned. Integrated circuit devices generally use only a few dopant species (e.g., boron, arsenic, phosphorus, and antimony) to form the requisite p-type and n-type doped regions of the device. However, many other species act as electrically active dopants, and are highly undesirable contaminants. These contaminants can have deleterious effects on the IC devices, such as increased junction leakage at concentrations well below 10.sup.13 cm.sup.-3.
Moreover, some less desirable contaminants segregate into the silicon substrate. This occurs when silicon is in contact with an aqueous solution, and the equilibrium concentration of the contaminants is higher in the silicon than in the solution. Moreover, some less desirable contaminants have very high diffusion coefficients. Consequently, introduction of such contaminants into any part of the silicon wafer may result in diffusion of the contaminants throughout the wafer, including junction locations where leakage may result.
Thus, liquid solutions for treating semiconductor wafers should have extremely low levels of metal ions. Preferably, the concentration of all metals combined should be less than 300 ppt (parts per trillion), and less than 10 ppt for any single metal. Even lower concentrations are desirable. Contamination by anions and cations should also be controlled. Some anions may have adverse effects, such as complexed metal ions which reduce to mobile metal atoms or ions in the silicon lattice.
Front end facilities typically include on-site purification systems for preparation of high-purity water (i.e., "deionized" or "DI" water). However, it is more difficult to obtain liquid process chemicals in the purities required.
c. Purity in Semiconductor Manufacturing
Undetected contamination of chemicals increases the probability for costly damage to a large quantity of wafers. The extreme purity levels required by semiconductor manufacturing are rare and unique among industrial processes. With such extreme purity requirements, handling of chemicals is undesirable (though of course it cannot be entirely avoided). Exposure of ultrapure chemicals to air (particularly in an environment where workers are also present) should be minimized. Such exposure risks the introduction of particulates into the chemicals, which can result in the contamination of those chemicals. Furthermore, shipment of ultrapure chemicals in closed containers is not ideal, since such containers increase the risk of contaminants being generated at the manufacturer's or at the user's site.
Since many corrosive and/or toxic chemicals are used in semiconductor processing, the reagent supply locations are commonly separated from the locations where front-end workers are present. Construction and maintenance of piping for ultra-high-purity (UHP) gases and liquids are well-understood in the semiconductor industry, and most gases and liquids can be transported to wafer fabrication stations from anywhere in the same building (or in the same site).
d. Uses of Nitric Acid in Semiconductor Processing
One of the important chemicals in semiconductor processing is nitric acid (HNO.sub.3). Nitric acid is useful in the wet etching of silicon. It is also used, for example, in chemical-mechanical polishing of interconnect metallization, in compounds for etching various heavy metals, and in defect etching.
e. Objects and Advantages of the Invention
To meet the requirements of the semiconductor processing industry and to overcome the disadvantages of the related art, it is an object of the present invention to provide a novel system for the on-site preparation of ultra-high-purity nitric acid, in which the nitric acid can be piped directly to a point of use. The system is very compact, and can be located in the same building as the point of use (or in an adjacent building), so that chemical handling can be avoided. As a result, low impurity levels on a semiconductor wafer surface can be achieved, resulting in better device characteristics and increased product yield.